Two-stroke lean burn gas engine with a silencer/catalytic converter

ABSTRACT

A low emissions 2-stroke natural gas fueled engine includes at least one cylinder with an exhaust port in communication with a silencer/catalytic converter unit. The unit has first and second volumes in communication with each other. The first volume dampens spurious exhaust pressure excursions and removes particulates in the exhaust. The second volume houses an oxidation catalyst for treating exhaust to reduce exhaust emissions. The engine oil has at most 10 ppm zinc content to reduce metal poisons contained in the exhaust prior to contact with the oxidation catalyst. The engine oil preferably has a very low ash content to minimize sulfur combustion components in the exhaust to reduce masking of the oxidation catalyst. The first volume preferably has a pressure relief valve set to relieve at a pressure greater than the maximum normal operating pressure of the engine to avoid excessive pressure excursions of the engine exhaust from damaging the oxidation catalyst.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a silencer/catalytic converter unit foruse with an internal combustion engine, in particular to a two-strokelean burn internal combustion engine using a normally gaseoushydrocarbon as fuel. The present invention further relates to atwo-stroke engine incorporating the silencer/catalytic converter unit.Additionally, the present invention relates to a method for convertingan existing 2-stroke engine to a low emissions 2-stroke engine. Finally,the present invention relates to a method for reducing carbon monoxide,formaldehyde and volatile organic compounds (VOC) emissions in theexhaust of a 2-stroke natural gas fueled engine.

BACKGROUND OF THE INVENTION

Two-stroke (alternatively referred to as two-cycle) engines have beenknown for many years and have been applied in a range of applications.One class of two-stroke engines is the class of engines operating on anormally gaseous hydrocarbon, most commonly natural gas, under lean burnconditions. Such engines are generally large, slow running engines of astationary design and find application in the driving of rotating andreciprocating equipment, such as compressors and electric generators.One example of commercially available engines is the Ajax® series ofengines manufactured and sold by the Cooper Energy Services division ofCooper Cameron Corporation. The Ajax engines are two-stroke engineshaving from one to four cylinders. When used to drive a compressor, theAjax engines are commonly employed in a configuration in which thecylinders of a reciprocating compressor are driven from the samecrankshaft as the cylinders of the engine.

Engines of this class generally operate at low speeds, that is speeds ofthe order of from several hundred to a thousand revolutions per minute.The engines are generally operated in a constant speed mode, in which asubstantially constant speed is maintained under a variety of engineloads. As the power demand placed on the engine is increased, thecombustion efficiency and performance of the engine improves.

Recent environmental regulations have been increasing the emphasis onthe importance of reducing the levels of partially burned fuelconstituents from the exhaust of stationary engines. These regulatedexhaust emissions consist of CO, NMHC, and formaldehyde (CH₂O). Anoxidizing catalyst in the exhaust stream will produce dramaticreductions in the levels of these emissions. Accordingly, there is aneed for a way to reduce carbon monoxide, formaldehyde and volatileorganic compounds (VOC) emissions from engines in this class.

One method of reducing the amount of such emissions in other types ofinternal combustion engines is to employ a catalytic converter in theexhaust system of the engine. The catalytic converter converts suchemissions in the exhaust gases to less harmful emissions before they areemitted to the atmosphere. However, that has proven more difficult inpractice. Previous industry experience with applying oxidizingconverters to 4-stroke natural gas fueled engines indicates satisfactoryresults relative to the removal efficiencies of the subject emissionsand the duration of operating time accumulated between catalyst cleaningand/or element replacement. However, previous tests of oxidizingcatalysts with 2-stroke natural gas fueled engines have demonstratedgood removal efficiencies for only short time periods. Therefore,currently available lean burn catalyst systems are limited to 4-strokeengine applications.

The majority of oxidation catalysts use a combination of platinum (Pt),rhodium (Rh), and palladium (Pd). Under the lean conditions that theseengines are run, there is excess oxygen present in the exhaust. Withexcess oxygen present, oxidation catalysts are effective at eliminatingcarbon monoxide, formaldehyde and VOC emissions.

All of the chemical reactions that occur in a catalyst occur at thesurface. So, any decrease in the surface area or the number of activesites available of the catalyst results in a decrease in theeffectiveness of the catalyst. The specific deactivation mechanismspresent in 2-stroke lean burn natural gas engines include selectivepoisoning and non-selective poisoning.

Selective poisoning occurs when a material reacts directly with thecatalytic material rendering it unable to function as a catalyst.Poisoning is generally a reversible process, which is treated by usingheat, washing or simply removing the poison from the exhaust stream.Sulfur from engine oil in the exhaust stream is a major contributor tocatalyst poisoning.

Non-selective poisoning is also referred to as masking or fouling. It isthe result of materials in the exhaust flow that accumulate on thecatalyst surface. Phosphorous compounds and other materials, which arecommon in lubricating oils and in partially burned combustion products,can be found on the surface of the catalyst.

Differences in catalyst performance are also affected by temperature.Higher temperatures increase catalyst efficiency and may impedepoisoning. The difference in temperatures is why 4-stroke natural gasfueled engines have been successfully outfitted with catalyticconverters and why there is still a need for them in 2-stroke naturalgas fueled engines. The difference in temperatures is due to thedifferences in engine design. Because of the scavenging process,2-stroke engines have cooler exhaust temperatures than 4-stroke enginesthat consequently hinder exhaust performance.

M. DeFoort et al. of Colorado State University reported these problemsand differences at the Gas Machinery Conference 2002 in Nashville, Tenn.on Oct. 8, 2002, in their paper entitled Performance Evaluation ofOxidation Catalysts for Natural Gas Reciprocating Engines. This paperdiscloses the use of a catalyst in an attempt to treat the exhausts from2-stroke and 4-stroke lean burn natural gas fueled engines. The catalystefficiency dropped from 95% to 80% for CO and from 75% to 45% forformaldehyde during the catalyst aging process for a large bore 2-strokeengine (about 200 hours). However, the results for the medium bore4-stroke engine were better due to the nearly 200 degree F. highercatalyst temperatures. The catalyst efficiency dropped from 99.2% to97.7% for CO and from essentially 100% to 67% for formaldehyde duringthe catalyst aging process (about 150 hours).

The specific 2-stroke engine used was a Cooper-Bessemer GMV-4TFstationary internal combustion engine having four cylinders with amanufacturer's sea level rating of 440 brake-horsepower (bhp) at 300rpm. The cylinders were 14 inches in diameter with a 14-inch stroke. Airwas delivered to the engine using a supercharged air delivery system.During the scavenging process, about half of the air supplied to theengine passed through the engine and was not trapped in the cylinder.The other half of the supplied air was trapped in the cylinder andparticipated in the combustion process. The catalyst was contained in ahousing having four units, each measuring 12″×16″×3″. The housing wasinserted in the exhaust line, but its location is not clear from thearticle since FIG. 6.1 showing its location was not published with thearticle.

M. DeFoort et al. analyzed the catalyst used with the 2-stroke engine.They found that the leading edge of the catalyst had three oxides notpresent in the trailing edge of the catalyst. These were oxides formedfrom copper (CuO), phosphorus (P2O5) and zinc (ZnO). Sulfur also playeda role in the deterioration of the catalyst. The elements copper,phosphorus and zinc, plus other elements such as iron and calcium,contributed to the deactivation of the catalyst, all of which are knowncatalyst poisons originating from engine lubricants and coolants. Inaddition, black soot was found on the leading edge of the catalyst.

In summary, M. DeFoort et al. concluded based on their results thatoxidation catalysts were not likely to be effective for large bore2-stroke lean burn engines. The oxidation catalyst showed clear signs ofpoisoning in a relatively short period of time (less than 250 hours)when compared to the expected lifespan of the catalyst

While catalytic converters for a 2-stroke engine are known in the art,their application has been limited to 2-stroke engines of much smallercapacity and operating at speeds far greater than those of the class ofengines addressed by the present invention See, for example, catalyticconverters disclosed in U.S. Pat. Nos. 6,277,784 (for small engines);and muffler/catalytic converter combinations disclosed in 4,867,270 (forportable hand tools); 5,866,859 (for portable work tools); 5,916,128(for small 2-stroke engine); 6,109,026 (for portable work tools);6,315,076 (for small engines); 6,403,039 (for small engines); and6,622,482 (for small engine applications).

To date because of the problems noted by M. DeFoort et al., suchcatalytic converter exhaust systems have not been applied to largecapacity 2-stroke lean burn engines operating on a normally gaseoushydrocarbon fuel and operating at speeds at or below about 1000 rpm.

Accordingly, there is a need for a solution to the problem of achievinglower carbon monoxide and formaldehyde emissions in the exhaust fromlarge capacity 2-stroke lean burn engines operating on a normallygaseous hydrocarbon fuel and operating at speeds at or below about 1000rpm, while maintaining a satisfactory level of catalyst efficiency andrequiring little maintenance over and above the existing maintenanceschedules.

SUMMARY OF THE INVENTION

Accordingly, the present invention satisfies this need by broadlyproviding a combination exhaust silencer and oxidizing catalyticconverter unit applied to a large capacity two stroke, lean burn (2SLB),gaseous fueled engine operating at speeds at or below about 1000 rpm andutilizing a lubricating oil with a zinc content of at most 10 ppm andwhich preferably has a very low ash content (less than 0.1 wt %).

In one aspect of the invention, there is provided a low emissions2-stroke natural gas fueled engine. The engine includes at least onecylinder with an inlet port and an exhaust port, and asilencer/catalytic converter unit, wherein the exhaust port incommunication with the silencer/converter. In one embodiment, an exhaustline is connected at one end to the exhaust port and at the other end tothe silencer/converter unit, thereby placing the exhaust port incommunication with the silencer/converter. In another embodiment, anexhaust line is connected at one end to the exhaust port and at theother end to an exhaust manifold with the silencer/converter unitconnected to an exhaust manifold, thereby placing the exhaust port incommunication with the silencer/converter. The silencer/catalyticconverter unit comprises a first volume and a second volume; wherein thefirst volume and the second volume are in communication with each other.The first volume is for dampening spurious exhaust pressure excursionsand for removing at least a portion of the particulates contained in anuntreated engine exhaust. The first volume can be one or more chambers.The second volume houses an oxidation catalyst for reducing emissions ina treated engine exhaust below the emissions in the untreated engineexhaust. The engine also has a lubricating engine oil having a zinccontent of at most 10 ppm, thereby reducing the metal poisons containedin the untreated exhaust prior to contact with the oxidation catalyst.Preferably, the lubricating engine oil has a zinc content of at most 5ppm. The lubricating engine oil is preferably produces very low ash,thereby minimizing the amount of sulfur combustion components containedin the untreated engine exhaust to reduce masking of the oxidationcatalyst. The first volume preferably has a pressure relief valve set torelieve at a pressure greater than the maximum normal operating pressureof the engine to avoid excessive pressure excursions of the engineexhaust from damaging the oxidation catalyst.

In another aspect of the invention, there is provided a method forconverting an original 2-stroke natural gas fueled engine to a converted2-stroke natural gas fueled engine having lower emissions. The methodcomprising providing the original 2-stroke natural gas fueled engineproducing an untreated engine exhaust containing particulates. Theoriginal engine has at least one or more cylinders with an inlet portand an exhaust port, a silencer in communication with the exhaust port;and an unmodified lubricating engine oil having a zinc content of atleast 300 ppm. The method also includes replacing the silencer with asilencer/catalytic converter unit. The silencer/catalytic converter unitincludes a first volume for dampening spurious exhaust pressureexcursions and removing at least a portion of the particulates containedin the untreated engine exhaust, and a second volume housing anoxidation catalyst for reducing emissions in a treated engine exhaustbelow the emissions contained in the untreated engine exhaust, whereinthe first volume and the second volume are in communication with eachother. The method further includes positioning the oxidation catalystwithin the second chamber such that the untreated engine exhaust has atemperature of at least 600 degrees F.; and replacing the unmodifiedlubricating engine oil with a low metals lubricating engine oil having azinc content of at most 10 ppm, more preferably at most 5 ppm, therebyreducing the metal poisons contained in the untreated engine exhaustprior to contact with the oxidation catalyst. Preferably, this methodalso includes the step of installing a pressure relief valve in thefirst volume set to relieve at a pressure greater than the maximumnormal operating pressure of the engine to avoid excessive pressureexcursions of the engine exhaust from damaging the oxidation catalyst.The low metals lubricating engine oil preferably produces a very low ashcontent (less than 01.%), thereby minimizing the amount of sulfurcombustion components contained in the untreated engine exhaust toreduce masking of the oxidation catalyst.

In yet another aspect of the present invention, there is provided amethod for reducing carbon monoxide, formaldehyde and VOC emissions inthe exhaust of a 2-stroke natural gas fueled engine. The method includeslubricating said engine with a lubricating engine oil composition havinga zinc content of at most 10 ppm, more preferably at most 5 ppm; feedingan untreated engine exhaust of the engine to a silencer/converter toproduce a treated engine exhaust; and positioning the oxidation catalystwithin the second chamber such that the untreated engine exhaust has atemperature of at least 600 degrees F. The silencer/converter has atleast a first volume for dampening spurious exhaust pressure excursionsand removing at least a portion of the particulates contained in theuntreated engine exhaust, and a second volume housing an oxidationcatalyst for reducing emissions in the treated engine exhaust below theemissions in the untreated engine exhaust. The first volume and thesecond volume are in communication with each other. The lubricatingengine oil utilized herein preferably produces very low ash (less than0.1%), thereby minimizing the amount of sulfur combustion componentscontained in the untreated engine exhaust to reduce masking of theoxidation catalyst. Preferably, the method includes the further step ofinstalling a pressure relief valve in communication with the firstvolume set to relieve at a pressure greater than the maximum normaloperating pressure of the engine to avoid excessive pressure excursionsof the engine exhaust from damaging the oxidation catalyst.

In further aspect of the present invention, there is provided asilencer/catalytic converter unit for a 2-stroke natural gas fueledengine. The silencer/catalytic converter unit includes an oxidationcatalyst for reducing carbon monoxide and formaldehyde emissions in anuntreated engine exhaust; a first volume for dampening spurious exhaustpressure excursions and removing at least a portion of the particulatescontained in the untreated engine exhaust; a second volume housing theoxidation catalyst for reducing emissions in a treated engine exhaustbelow the emissions in the untreated engine exhaust; and a pressurerelief valve in communication with the first volume set to relieve at apressure greater than the maximum normal operating pressure of theengine exhaust to avoid excessive pressure excursions of the engineexhaust from damaging the oxidation catalyst. The first volume and thesecond volume are in communication with each other. The oxidationcatalyst is positioned within the second chamber such that duringoperation of the engine the untreated engine exhaust has a temperatureof at least 600 degrees F. at that position. Preferably, at least oneexhaust flow pipe provides the communication between the first andsecond volumes. Each of the at least one exhaust flow pipes has acatalyst facing end which is closest to the first catalyst face of theoxidation catalyst. The distance between the catalyst facing end and thefirst catalyst face is sufficient to provide a substantially uniformflow of the untreated exhaust upon contact across the first catalystface during engine operation. This enhances the utilization of theoxidation catalyst.

Catalyst:

The oxidation catalyst reduces the concentration of carbon monoxide,formaldehyde and VOC's in the engine exhaust. Such catalysts arecommercially available, for example, from EAS, Inc., Crystal Lake, Ill.,and Johnson-Matthey, Malvern, Pa.

The U.S. EPA rule that was promulgated in March, 2004 requires COremoval efficiency to be at 58% or higher for two stroke, gas fueledengines. Preferably, the catalysts are selected sized to produce atleast a 70% removal of CO and a 55% removal of formaldehyde. This willallow for a gradual degradation of catalyst efficiency over asufficiently long period of time between periods of catalystregeneration or replacement, preferably coinciding with other scheduledengine maintenance.

An example of a particularly preferred catalyst is provided by EAS, Inc.with the tradename ADCAT™ catalyst. This catalyst uses platinum on astainless steel honeycomb substrate. After our experiments with thiscatalyst, a standardized size for the catalyst element was defined foruse on all Ajax® engine models. Each Ajax® engine will use one of thesecatalyst elements per power cylinder. These elements are 12.5″ wide×34″long×3.7″ thick. The face surface area and the thickness for thecatalyst were determined from our tests and based on the flow area andan estimate of the exhaust residence time in the catalyst needed toproduce the required emissions removal efficiencies.

The catalytic converter must provide the required emissions removalefficiencies throughout the normal engine operating range, which is 265RPM to 440 RPM and from 50% to 100% torque. The above range extends from60 to 200 BHP per power cylinder. The converter is required to operateproperly with the wide range of fuel gases, which are typically used atvarious field sites. This variety includes fuels having lower heatingvalues (LHV) from 450 to 1500 BTU/ft³. On the lower end of the LHVrange, these fuels contain high quantities of inert gases like CO₂ andN₂. On the upper end of the LHV range, these fuels contain highquantities of the heavier hydrocarbons, like propane, butane, and smallamounts of pentane.

The primary areas of focus for our experiments were: (1) operation nearthe design rating, which is 200 BHP per cylinder, and (2) use ofpipeline quality fuel gas, which consists mainly of methane and has anLHV of 950 BTU/ft³.

Catalyst Retaining Rack:

A catalyst element retaining rack is located inside the second chamberor volume of the silencer/catalytic converter unit (See FIGS. 3 and 6).This concept results in providing exhaust silencing while also servingas a catalyst housing. It also assures that the catalyst operatingtemperatures are high enough to achieve large removal efficiencies forthe exhaust emissions.

Catalyst Surface Area and Residence Time:

Based on our testing to date, we expect that the engine will operate formore than 4000 hours before regeneration of the catalyst elements isneeded. Measurement of CO before and after the catalyst is the preferredmethod for determining when the catalyst needs to be regenerated. At theend of our 500-hour lab test, the CO removal efficiency was about 92%.We expect that more than six months of continuous operation can becompleted before the CO removal efficiency drops to the 58% level.

Preferably, there is one catalyst element per power cylinder of theengine. The catalyst element in one embodiment is 12.5″ wide×34″long×3.7″ thick. Therefore, the overall width of a set of catalystelements is equal to the number of power cylinders times the width of asingle catalyst element, which in this case the width is 12.5″.

This catalyst element weighs 45 lbm. As a result, these catalystelements can be installed without the crane and installation/removaldevice. However, in an earlier embodiment, a single round catalystelement was used for the lab tests and it weighed more than 200 lbm.This larger and heavier catalyst element required the use of a hoist andinstallation/removal device shown in FIG. 10.

Based on our tests, we have determined a direct relationship betweenexhaust flow and catalyst surface area and between exhaust flow andcatalyst thickness. If less catalyst is used, then the emissions removalefficiencies are inadequate. If more catalyst is used, then the removalefficiencies for CO, VOC's and H₂CO are increased, but the NO_(X)increase across the catalyst becomes unacceptable.

The total face surface area for the catalytic elements is preferablyfrom about 20 to about 30 sq. in., more preferably from about 24 toabout 28 sq. in., for each 100 actual ft³/min of exhaust flow, where“actual” means that the flow is referenced to the exhaust temperature atthe catalyst. For the EAS catalyst tested, the preferred total facesurface area for the catalytic elements is from about 24 to about 28 sq.in. for each 100 actual ft³/min of exhaust flow.

The effective residence time for exhaust to spend in the catalyst ispreferably from about 0.025 to about 0.050 seconds, more preferably fromabout 0.030 to about 0.040 seconds, and yet more preferably from about0.031 to about 0.037 seconds. For the EAS catalyst tested, the effectiveresidence time was preferably from about 0.031 to about 0.037 seconds.The term “effective residence time” is based on the thickness of thecatalyst element. Actual residence time would be slightly higher becausethe path traveled through the catalyst is slightly longer than astraight line.

Relative to the EAS catalyst tested, other catalysts can have the sameor different amounts of noble metal and the same or different exposedareas of the catalyst material to the exhaust passing through thecatalyst element, depending on their internal structure. The abovepreferred ranges for the EAS catalyst would be good initial estimatesfor other catalysts, but routine testing of the catalysts can be used todetermine their optimum face surface area and residence time factors.

Catalyst Location:

The preferred location for the catalyst is determined from the followingfactors:

a. Exhaust Tuning:

The exhaust is tuned to maximize the power output from a two-strokeengine. This involves the length of the exhaust pipe from the powercylinder to the end of the exhaust pipe. As is known to those skilled inthe art, the exhaust pipe length is dependent on the swept volume forthe power cylinder, the crank angle at which the exhaust ports open, andthe rated speed for the engine. The preferred exhaust pipe length forthe Ajax 2801LE, 2802LE, 2803LE, & 2804LE engines is 15′-6″.

b. Volume of the First Chamber of the Silencer/Converter:

The volume of the first chamber or volume of the silencer/converter is afunction of the swept volume of the power cylinders (enginedisplacement). The volume of the first chamber of the silencer/converteris preferably equal to the number of exhaust pipes connected to thatchamber times the swept volume for one cylinder times about 18, which islarge enough to contain the exhaust from about 17 to 19 revolutions ofthe engine. This is the preferable volume to damp out the exhaustpulsations without upsetting the tuning effects gained from the tunedexhaust pipe.

c. Temperature:

To achieve acceptable emissions removal efficiencies, the oxidationcatalyst must be in a location where the exhaust temperature is about600° F., or higher. Therefore, the silencer/converter of the presentinvention is designed and installed to position the catalyst relative tothe engine exhaust port such that this temperature is achieved.

Baffle and Flow Pipes:

A system of internal baffles and pipes is arranged inside thesilencer/converter so that the catalyst element is protected frommasking or fouling from liquid or particulate carryover into theexhaust. This system also protects the catalyst from sudden highpressure excursions and pulsations in the exhaust system (see FIGS. 3and 6).

The main design feature that is pertinent to achieving a satisfactorycatalyst life is based on causing the changes to the direction and tothe flow rate of the exhaust prior to entering the catalyst. Thisfeature results in minimal carry-over of liquid droplets andparticulates to the catalyst.

The prior art silencers do not include considerations of limiting thecarry-over of liquid droplets and particulates and they do not includeany provision for the installation of a catalyst.

The new silencer/converter designs of the present invention have manysimilarities with the prior art silencer design by necessity since bothdesigns perform the exhaust silencing function. However, thesilencer/converter of the present invention includes additionalfeatures, which are described herein, relating to the incorporation andprotection of the oxidation catalyst.

A baffle and flow pipes must be place in the silencer/converter ahead ofthe catalyst to protect the catalyst from liquid and particulatecarry-over from the exhaust pipes.

A sufficient distance between (1) the exits of the flow pipes from thefirst chamber or volume into the second chamber or volume and (2) thecatalyst face is preferred to allow the exhaust flow to be distributedevenly across the face of the catalyst. For example, with asilencer/converter cross sectional flow area of 8-12 ft² and an exhaustflow of about 1400-1600 actual ft³/min per engine cylinder, thisdistance is preferably a minimum of about 1-½ feet.

The volumes of the chambers in the silencer/converter are dependent onthe swept volume of the power cylinders. The flow areas of the pipesbetween these chambers are dependent on the total exhaust flow rate.Relative to protecting the catalyst from masking due to liquid dropletsor particulates, the main functions of the baffles and pipes are to:

-   -   a. Reduce the flow velocity as the exhaust enters the first        volume chamber causing liquid and solid constituents to drop out        of the exhaust stream.    -   b. Produce some radical changes in flow direction to promote the        drop out process.    -   c. Increase the exhaust flow velocity as the exhaust passes        through the pipes between the volume chambers.    -   d. Substantially reduce the amplitude of the exhaust pressure        pulsations before the exhaust reaches the catalyst.    -   e. Provide an even distribution of exhaust flow across the face        of the catalyst.

The exhaust velocity in the exhaust pipes at the point of entrance tothe silencer/converter is 2900-3200 feet per minute at the design ratedengine speed of 440 RPM with our standard production engines, which havea displacement volume of 2827 in³ per cylinder. The exhaust velocity atthe input face of the catalyst for the above conditions ranges from 500to 600 feet per minute.

From the perspective of designing a successful oxidizing converter, thetwo important factors relating to chamber volumes are:

-   -   (1) A first chamber volume that will damp out the exhaust        pulsations, and    -   (2) Enough length between the exits of the flow pipes to the        second chamber to the catalyst face to allow the exhaust flow to        be distributed evenly across the face of the catalyst. For        example, with a silencer/converter cross sectional flow area of        8-12 ft² and an exhaust flow of about 1400-1600 actual ft³/min        per cylinder, this length would be a minimum of about 1-½ feet.

The relationships of the other chamber volumes and flow areas betweenthe chambers are important for exhaust noise silencing, but not for thecatalyst application. These relationships are commonly used bymanufacturers of exhaust silencers.

The silencer/converter system has been designed in both the vertical andhorizontal configurations to cover the possible variations in fieldsites (See FIGS. 3 and 6).

Pressure Relief Valve:

A pressure relief valve (or valves) is (are) used to protect thecatalyst elements from sudden high pressure excursions in the exhaustsystem (See FIGS. 3 and 6).

A pressure relief valve is preferably placed in the first volume chamberto protect the catalyst from high-pressure excursions in the exhaustsystem that are caused by occasional firing into the exhaust ports. Someincorrectly refer to these events as backfires.

Preferably, there is one relief valve per exhaust inlet or pair ofexhaust inlets. The incoming exhaust is preferably aimed towards thepressure relief valve, which is generally located opposite from theexhaust inlets. The relief valve setting must accommodate the normalexhaust pressure fluctuations exhibited in the first volume chamber, butit must relieve immediately when a higher pressure pulse than normaloccurs. The pressure relief valve needs to relieve at the lowestpressure that allows adequate safety margin from the normal engineoperating conditions. By recording the exhaust pipe pressures asfunctions of time during normal operating conditions, the highest normalpressure pulse was determined to be about 3 psig. Allowing for about aone to two psi safety margin, the relief valve is set at 4-5 psig toprotect the catalyst from a higher pressure excursion.

Lubricating Oil:

Unlike four stroke engines, two stroke engines must have lubricating oiladded to the power cylinders. This oil is mixed with the fuel forgasoline engines and is directly injected into the power cylinders fornatural gas fueled engines. Two stroke gas engine operation tends toform various deposits such as varnish, sludge and an ash residue thatremains after the oil is burned during operation. Addingdetergent/dispersant additives controls the varnish and sludge. However,these detergent/dispersant additives tend to leave a gray, fluffy ashresidue after the oil has been burned. This ash residue is made LIP ofmetal sulfates from such additives as barium, calcium, phosphorus, zinc,magnesium and boron, which deactivate the exhaust catalyst by formingglassy-amorphous deposits, which prevent the exhaust gas from reachingthe active surfaces of the catalyst.

The lubricating oil used in the present invention for the 2-strokeengine power cylinders is formulated to minimize the type of oiladditives that would degrade the catalyst efficiency. The power cylinderlube oil is formulated so that the zinc content was reduced from about300 ppm present in prior art oils to at most less than 10 ppm,preferably at most 5 ppm. Other metals that poison the catalyst are alsopreferably at low levels to avoid poisoning of the active sites of thecatalyst.

Such lube oils are formulated to reduce the metallic additives whileincreasing some of the non-metallic additives to provide acceptablelubricating properties for the 2-stroke natural gas fueled engine. Thelube oil provided for the experiments conducted herein was formulated byExxonMobil, which modified its Mobil Pegasus Special 10 W-40 with thespecial additives, which reduce the zinc content to less than 5 ppmwhile maintaining the required lubrication properties.

Preferably, the lube oils have a very low ash content. The term “ash”refers to a metal-containing compound wherein the metal can be zinc,sodium, potassium, magnesium, calcium, lithium, barium, and the like, asmeasured by ASTM D874. Ash can also contain sulfur in the form ofsulfated ash. The term “very low ash content” refers to less than 0.10wt % ash content in the lubricating oil composition. Very low ash lubeoils reduce the sulfur oil combustion products, which poison theoxidation catalyst by masking the catalyst active sites.

Other lube oils for 2-stroke natural gas engines are commerciallyavailable, which have low metals content. Examples of these includeMysella 40 available from Shell Lubricants (0.01 sulfated ash % by mass,0 zinc content % wt, 0.025 phosphorous % wt, 0 calcium % wt) and ChevronHDAX ashless gas engine oils (nil sulfated ash % wt, less than 10 ppmzinc content, 670 ppm phosphorous). As earlier noted, phosphorous andcalcium also poison catalyst active sites.

If the lube oil were not reformulated to have a low zinc content and tohave a very low ash content (according to ASTM method D874), then theinitial emissions removal efficiencies for the catalyst would be aboutequal to the efficiencies measured during our experiments, but thecatalyst would be poisoned and masked quicker. Since the emissionsremoval efficiencies would be expected to fall to unacceptable levels inless than six months of operation, the effects of operating without thereformulated oil were not measured. As noted earlier, metallic additivesin the oil would cause catalyst degradation problems.

Catalyst Installation Rig (Optional Equipment):

An installation and removal rig for the catalyst element with thecatalyst rack was designed so that a catalyst element and catalyst rackcan be lifted, inserted into the converter housing, and extracted fromthe housing while working from the ground level (See FIG. 10). The righas a tray with four lifting points. A heavy catalyst element with itscatalyst rack is placed on the tray. Chains are attached to the liftingpoints. A hoist or block and tackle arrangement with a lifting cable orchain is attached to the chains attached to the tray. Once the tray islevel with the access flange for the catalyst retainer rack housing anddrawer slide in the second or catalyst chamber, the tray is secured tothe access flange. Attached to tray opposite the access flangeattachment is a rotatable wheel having female screw portion thatreceives an elongated male threaded rod that is attached on one endthereof to the catalyst rack at a point opposite to the access flange.The rotatable wheel is rotated using a sprocket and chain assembly orwith a motor assist to push the catalyst rack in through the opening inthe access flange onto the drawer slide or to withdraw the catalystretainer rack from the second or catalyst chamber. Once the catalystrack is fully inserted and resting on the drawer slide, the threaded rodis released from its attachment point on the catalyst rack and theaccess cover door is replaced on and attached to the access flange.

Materials of Construction:

Nearly all of the silencer/converter is constructed of 10-gauge sheetsteel, but there are several places where an inner shell of the samematerial is used to produce the appropriate sound deadening qualities.

The frames for the catalyst elements and portions of the retaining rackfor the elements are preferably fabricated with stainless steel.

The gaskets for the catalyst and for the access cover door are producedwith a high temperature fiberglass material that provides good sealingup to about 900° F.

New or Retrofits:

This combination silencer/converter is preferably designed to beinterchangeable with prior art exhaust silencers and can therefore beused on a new engine unit or as a field retrofit on an existing engineunit. In a retrofit situation, the engine lubricant would be changed toa lubricating engine oil having a zinc content less than 10 ppm,preferably less than 5 ppm, and is preferably also very low ash content.Such lubricating engine oils are used in conjunction with thesilencer/converter unit of the present invention to extend the durationfor achieving satisfactory emissions removal efficiencies for thecatalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation of a prior art internal combustionengine with a silencer.

FIG. 2 is a schematic side elevation of an embodiment of an internalcombustion engine with a silencer/catalytic converter according to thepresent invention.

FIG. 3 is a schematic side elevation of a vertical embodiment of asilencer/catalytic converter according to the present invention.

FIG. 4 is a side view of the catalyst retainer rack of FIG. 3 with twocatalyst elements.

FIG. 5 is a top view of a section of the catalyst chamber of FIG. 3showing the catalyst retainer rack, its gasket, and the shoulder locatedwithin the catalyst chamber for seating the gasket.

FIG. 6 is a schematic side elevation and a partial cross-section of ahorizontal embodiment of a silencer/catalytic converter according to thepresent invention.

FIG. 7 is a schematic end elevation of the silencer/catalytic converterin FIG. 6 showing the relative placement and angle of the exhaust inletand the relief valve.

FIG. 8 is a side view of the catalyst retainer rack of FIG. 6 with fourcatalyst elements.

FIG. 9 is a top view of a section of the catalyst chamber of FIG. 6showing the catalyst retainer rack, its gasket, and the shoulder locatedwithin the catalyst chamber for seating the gasket.

FIG. 10 is a schematic top view of an embodiment of an optional catalystinstallation and removal system according to the present invention.

FIG. 11 is a graph of catalyst efficiency curves showing previousindustry results using dashed lines and the results obtained during theexperiment reported herein.

DETAILED DESCRIPTION

Referring to FIG. 1, a prior art stationary two-stroke or two-cycleinternal combustion engine system 500 is shown having from one to fourcylinders, with only one cylinder 30 schematically shown. The cylinder30 has an inlet port 36 and an exhaust port 38. A gaseous hydrocarbonfuel is fed into each cylinder 30 at the appropriate point in theengine's cycle via line 32 in fluid communication with the inlet port36. A source of lubricating engine oil is provided to the engine vialine 34. Details of the engine have been omitted from FIG. 1 for thesake of clarity. Stationary natural gas fueled 2-stroke enginestypically operate at constant speeds in the range of from 200 to 1000rpm, more typically 250 to 500 rpm.

In operation, a piston reciprocates within each cylinder 30 of thestationary engine. As the piston descends within the cylinder movingaway from the cylinder head, it opens an inlet port 36, through which agas or a mixture of gases is admitted and flows into the cylinder 30. Atthis time, the cylinder 30 is filled with gases which are products ofcombustion. In certain designs of engine, a mixture of gaseous fuel andair is admitted into the cylinder 30 through the inlet port 36 at thistime. In other designs of engine, such as the Ajax® engines referred toabove, air alone is admitted to the cylinder 30 through the inlet port36. At the same time that the inlet port 36 is open, the descendingpiston also uncovers an exhaust port 38, through which the burnt gasesleave the cylinder 30 via exhaust pipe 40, to form the exhaust gas ofthe engine. The action of the freshly charged gases entering thecylinder 30 through the inlet port 36 serves to assist with forcing theburnt gases out of the exhaust port 38, referred to as scavenging. Theexhaust gases travel through the exhaust pipe 40, and then through thesilencer 46 and exhaust stack 48.

Referring now to FIG. 2, there is shown an engine system 600 accordingto the present invention. System 600 requires the use of a stationarynatural gas fueled 2-stroke engines that typically operate at constantspeeds in the range of from 200 to 1000 rpm, more typically 250 to 500rpm. These engines operate on a normally gaseous hydrocarbon as itsfuel, for example, methane, ethane, propane and butane. System 600differs from prior art system 500 in that the lubricating engine oil vialine 34 is changed to be a lubricating engine oil via line 52 which hasat most 10 ppm zinc and is preferably very low in ash content.Additionally, the silencer 46 and its exhaust stack 48 are changed to asilencer/catalyst converter unit 50 according to the present inventionwith its exhaust stack 54 to reduce the emissions in the exhaust. Thesilencer/converter unit 50 can be in vertical or horizontal embodiments.An example of a vertical embodiment is unit 100 and of a horizontalembodiment is unit 200, which are discussed further below. Though notshown, in another embodiment, an exhaust manifold can also be used. Forexample, the exhaust pipe 40 is connected to the exhaust manifold 42(instead of directly to the silencer/catalyst converter unit 50) and asilencer line 44 is connected on one end to the exhaust manifold 42 andon the other end to the silencer/catalyst converter unit 50.

Referring now to FIG. 3, there is shown a schematic side elevation of avertical embodiment of a silencer/catalytic converter unit 100 accordingto the present invention. Unit 100 has an outer shell 101 with a lowerhead 132 and an upper head 133 enclosing a first volume chamber 134, asecond volume chamber 136, and a third volume chamber 138 verticallypositioned relative to each other. A first baffle 102 separates thefirst volume chamber 134 and the second volume chamber 136. A secondbaffle 104 separates the second volume chamber 136 and the third volumechamber 138. The second chamber 136 has a catalyst holding area 116having a catalyst access door 118.

Referring now to FIG. 4, there is shown a side view of a section of acatalyst holding area 116 of the catalyst or second volume chamber 136.The catalyst holding area 116 includes the catalyst retainer rack 128that rides on the rack slide 129, a gasket 130 for the catalyst rack128, and a shoulder 126 located within the catalyst chamber 136 forseating the gasket 130. Any suitable means for seating the catalyst rack128 against the shoulder 126 with the gasket 130 between them can beused, for example, a cam device (not shown). An access door 118 is usedto access the catalyst rack 128 for removing or installing the catalystelements 124. A top view of the catalyst retainer rack 128 with twocatalyst elements 124 is shown in FIG. 5.

Referring again to FIG. 3, the exhaust from the engine enters the firstvolume chamber 134 through exhaust inlet 110. The number of exhaustinlets 110 depends on the number of cylinders in the engine, typicallyone for each cylinder or a pair of cylinders. A relief valve 114 isgenerally positioned opposite the exhaust inlet 110. Due to the baffle102 and changing the direction of flow of the exhaust within the firstvolume chamber 134, liquid and solid particulates are at least partiallyremoved from the exhaust. These collect in the lower silencer head 132.A drain line and valve assembly 112 is attached to the bottom of thelower silencer head 132 to allow removal of any accumulated liquid andparticulate solids.

The volume of the first volume chamber 134 is sufficient to dampenspurious pressure excursions or pulsations to avoid damage to thecatalyst elements 124. The exhaust then exits the first volume chamber134 through flow pipes 106 into the second volume chamber 136. Theleading face of the catalyst elements 124 are spaced from the exit ofthe flow pipes 106 to allow a uniform flow of the exhaust across theface of the catalyst elements 124 to more fully utilize the availablecatalyst active sites in the catalyst elements 124.

After the exhaust passes through the catalyst elements 124, the exhaustexits the second volume chamber 136 into the third volume chamber 138through flow pipes 108. The exhaust then exits the third volume chamber138 through flow pipe 120, which enters the exhaust stack 122.

The volume of the second volume chamber 136 and the volume of the thirdvolume chamber 138, along with the volume of the first volume chamber134, are to produce the silencing effects of the unit 100.

Referring now to FIG. 6, there is shown a schematic side elevation inpartial cross-section of a horizontal embodiment of a silencer/catalyticconverter unit 200 according to the present invention. Unit 200 has anouter shell 140 with a first outer head 142 and a second outer head 143enclosing a first volume chamber 174, a second volume chamber 175, athird volume chamber 176 horizontally positioned relative to each otherwith the third volume chamber 176 between the first and second volumechambers 174 and 175, respectively. A fourth volume 178 is located abovethe third volume chamber with a fifth volume chamber 179 above thefourth volume chamber. A first baffle 146 separates the first volumechamber 174 and the third volume chamber 176. A second baffle 147separates the second volume chamber 175 and the third volume chamber176. A third baffle 148 separates the third volume chamber 176 and thefourth volume chamber 178. The fourth volume chamber 178 has a catalystholding area 164 having a catalyst access door 165. A fourth baffle 150separates the fourth volume chamber 178 and the fifth volume chamber179.

Referring now to FIG. 8, there is shown a side view of a section of acatalyst holding area 164 of the catalyst or fourth volume chamber 178.The catalyst holding area 164 includes the catalyst retainer rack 168that rides on the rack slide 169, a gasket 170 for the catalyst rack168, and a shoulder 172 located within the catalyst chamber 178 forseating the gasket 170. Any suitable means for seating the catalyst rack168 against the shoulder 172 with the gasket 170 between them can beused, for example, a cam device (not shown). An access door 165 is usedto access the catalyst rack 168 for removing or installing the catalystelements 166. A top view of the catalyst retainer rack 168 with fourcatalyst elements 166 is shown in FIG. 9.

Referring again to FIG. 7, the exhaust from the engine enters the firstvolume chamber 174 through exhaust inlets 158A and 158B. The exhaustfrom the engine also enters the second volume chamber 175 throughexhaust inlets 158C and 158D. In this embodiment, the unit 200 is for a4-cylinder engine. The number of exhaust inlets 158 depends on thenumber of cylinders in the engine, typically one for each cylinder or apair of cylinders. In this embodiment, the engine has 4 cylinders andthere are four exhaust inlets 158A, 158B, 158C and 158D. A relief valve162 is generally positioned opposite the exhaust inlets 158. In thisembodiment, there are two relief valves 162—one for each of the firstvolume chamber 174 and the second volume chamber 175. Each relief valve162 is positioned generally opposite from and between the respectiveexhaust inlets Therefore, one relief valve 162 is generally opposite andbetween the exhaust inlets 158A and 158B; and the other relief valve 162is generally opposite and between the exhaust inlets 158C and 158D. Whenlooking down the long axis L of the unit 200, the angle A between theaxis R of the relief valve 162 and the axis E of the exhaust inlet 158is at most 45 degrees.

Due to the baffles 146, 147 and 148, plus changing the direction of flowof the exhaust within the first, second and third volume chambers 174,175 and 176, liquid and solid particulates are at least partiallyremoved from the exhaust. These collect in the bottom of chambers 174,175 and 176. A drain line and valve assembly such as assembly 112 shownin FIG. 3 are added to the bottoms of each of chambers 174, 175 and 176to allow removal of any accumulated liquid and particulate solidstherein.

The volumes of chambers 174, 175 and 176 are sufficient to dampenspurious pressure excursions or pulsations to avoid damage to thecatalyst elements 166. The exhaust exits the first volume chamber 174through flow pipes 152 into the third volume chamber 176. The exhaustexits the second volume chamber 174 through flow pipes 153 into thethird volume chamber 176. The exhaust exits the third volume chamber 176through flow pipes 154 into the catalyst chamber or fourth volumechamber 178. The leading face of the catalyst elements 166 are spacedfrom the exit of the flow pipes 154 to allow a uniform flow of theexhaust across the face of the catalyst elements 166 to more fullyutilize the available catalyst active sites in the catalyst elements166.

After the exhaust passes through the catalyst elements 166, the exhaustexits the fourth volume chamber 178 into the fifth volume chamber 179through flow pipes 156. The exhaust then exits the fifth volume chamber179 through the exhaust stack 160, which optionally has a flange asshown herein for attaching to a stack extension (not shown).

The volume of the fourth volume chamber 178 and the volume of the fifthvolume chamber 179, along with the volume of chambers 174, 175 and 176,are to produce the silencing effects of the unit 200.

Referring now to FIG. 10, there is shown a top perspective elevation ofan embodiment of a catalyst installation and removal system 300according to the present invention used on a vertical unit 100′, whichis similar to unit 100, except that a single round catalyst element 124′with a round catalyst rack 128′ is used instead. The system 300 wasdesigned so that the catalyst element 124′ and catalyst rack 128′ can belifted, inserted into the converter housing 302, and extracted from thehousing 302 while working from the ground level. The system 300 has atray 304 with four lifting points 306. A heavy catalyst element 124′with its catalyst rack 128′ is placed on the tray 304. Chains 308 areattached to the lifting points 306. A hoist or block and tacklearrangement with a lifting cable or chain (not shown) is attached to alifting eye 310 to which the chains 308 are attached. Once the tray 304is level with the access flange 312 for the catalyst retainer rackhousing 302 and drawer slide in the second or catalyst chamber, the tray304 via its attachment ears 314 is secured to the access flange 312.Attached to tray 304 opposite the access flange 312 attachment is arotatable wheel 316 on a mount 317, wherein the wheel 316 has a femalescrew portion that receives an elongated male threaded rod 318 that isattached on one end 320 to an attachment mount 322 on the catalyst rack128′ at a point opposite to the access flange 312. The rotatable wheel316 is rotated using a sprocket and chain assembly or with a motorassist to push the catalyst rack 128′ in through the opening in theaccess flange 312 onto the drawer slide 129 (see FIG. 4) or to withdrawthe catalyst retainer rack 128′ from the second or catalyst chamber 136(see FIG. 3). Once the catalyst rack 128′ is fully inserted and restingon the drawer slide 129, the threaded rod 318 is released from theattachment mount 322 on the catalyst rack 128′ and the access cover door118 (see FIG. 3) is replaced on and attached to the access flange 312.

Experiment:

A vertical silencer/catalytic converter unit according to the presentinvention was installed on an Ajax® DPC-2802LE engine in the Ajax R & DLab, and was tested for nearly 500 hours with the engine operating atfull speed, nearly full torque, and close to the full rated BHP.

The Ajax® DPC-2802LE engine is a two-stroke, lean burn, natural gasfired engine. It has 2 power cylinders, each with a bore of 15 inchesand a stroke of 16 inches. The engine speed is 265 to 440 rpm. The priorart silencer was replaced with a vertical silencer/converter like thatshown in FIG. 3. However, the catalyst retaining rack was round as wasthe single catalyst element as shown in FIG. 10. The catalyst was about3½ feet in diameter, 3.7 inches thick and weighed about 200 lbm. Thecatalyst was ADCAT® catalyst from EAS, Inc. This catalyst uses platinumon a stainless steel honeycomb substrate. A catalyst lifting rig asshown in FIG. 10 was used to lift and install or remove the catalyst andcatalyst rack from the silencer/converter. The overall height of thesilencer/converter unit without the exhaust stack was about 16 feet witha diameter of about 3½ feet. The volume of the first chamber 134 wasabout 72 cu. ft. The volume of the second chamber 136 was about 42 cu.ft. The volume of the third chamber 138 was about 31 cu. ft. Thedistance between the exit of the flow pipe 106 and the leading face ofthe catalyst element 116 was about 1½ feet. There were 2 exhaust inlet110 from the exhaust pipe(s) connected to the exhaust ports of theengine. The conventional lubricating engine oil that the engine used hadabout 300 ppm zinc. This oil was replaced with a modified Mobil PegasusSpecial 10 W-40 formulated by ExxonMobil to have less than 5 ppm zincand had an ash content of less than 0.1 wt %. The average exhausttemperature at the catalyst location in the silencer/converter was about640 degrees F.

Initial performance for this invention achieved 93% removal of the COemissions and 91% removal for the formaldehyde. Although theseefficiencies were better than expected, a major feature of thisinvention is to prevent premature degradation of the catalyst removalefficiencies. As reported by DeFoort et al, their tests of oxidizingcatalysts with 2SLB engines indicated that the removal efficienciesdropped to unacceptable levels within less than two weeks.

Catalyst efficiency curves are presented in FIG. 11. These curvesexpress the removal efficiencies vs. hours of operation for thisinvention as compared to those reported by DeFoort et al., who usedoxidizing converters on 2SLB engines.

Standard exhaust emissions levels for Ajax® LE engines operating withpipeline quality fuel at the design rating with site elevations lessthan 1500 FASL (feet above sea level) are:

-   -   NO_(X)=2.0 gm/BHP-hr    -   CO=1.2 gm/BHP-hr    -   NMHC=1.2 gm/BHP-hr    -   H₂CO=0.29 gm/BHP-hr

This catalyst and silencer/converter have been tested for nearly 500hours at the design rating for the engine, and the oxidizingefficiencies were almost equal to the efficiencies recorded at the startof the tests.

Our Lab tests of the EAS oxidizing catalyst with the Ajax® DPC-2802LEengine included 430 hours with the full catalyst flow area, followed by51 hours with 60% of the flow area. Our reasons for blocking 40% of theflow area were (1) to resolve the problem with NO_(X) increase acrossthe catalyst and (2) to determine the amount of catalyst needed forfield applications.

The results from the Lab tests are in the following Table, whichincludes five columns expressing the average engine data and catalystdata during five time periods of the testing, which are defined in theaccumulated hours row of the spreadsheet.

The main conclusions from this testing are:

-   -   1. The CO and H₂CO removal efficiencies are substantially        maintained over these 500 hours.    -   2. Degradation of the removal efficiencies was minimal during        the 481 hours of testing. These efficiencies dropped by only        2-3% during this phase of the test project.    -   3. The NO_(X) increase across the catalyst was unacceptable        during the first 430 hours of testing. This increase averaged        23% during this time. The source for the nitrogen that was being        converted to NO_(X) was the nitrogen containing compounds in the        lube oil. Mobil reports that it is not viable to reduce these        compounds by a significant amount.    -   4. With 40% of the catalyst flow area blocked off, the NO_(X)        increase is acceptable. During the last 30 hours of testing,        this increase averaged less than 5%. Blocking 40% of the        catalyst flow area had minimal effects on the removal        efficiencies for the CO and H₂CO.    -   5. Though emissions removal efficiencies are expected to degrade        over time, removal efficiencies which should be achievable for        at least six months are expected to be:        -   CO—70% reduction

H₂CO—60% reduction. TABLE Average Data During 481 Hours of CatalystOperation Catalyst Type & EAS- EAS- EAS- EAS- EAS- Flow Area 100% 100%100% 60% 60% Hours Accumulated with 0-60 60-231 231-430 430-451 451-481Catalyst Engine Speed 440 440 440 440 440 BHP 361 352 352 352 384 (% ofFull Rated BHP) (94%) (92%) (92%) (92%) (100%) Exhaust Flow (SCFM) 16701670 1660 1650 1650 Exhaust Temp. 648 645 640 650 670 (° F. beforecatalyst) Exhaust Temp. 608 608 600 612 636 (° F. after catatalyst) %Oxygen in the Exhaust 14.2 14.3 14.2 14.3 13.8 Exhaust Press. atSilencer 3.3 3.2 3.2 3.2 3.65 /Converter Inlet (“H₂O) Pressure Dropacross the 0.4 0.5 0.5 0.55 0.9 Catalyst (“ H₂O) CO (gm/BHP-hr Before1.4 1.4 1.3 1.4 1.7 Catalyst) CO (gm/BHP-hr After 0.07 0.10 0.09 0.110.14 Catalyst) CO (ppm Before Catalyst) 153 152 143 150 187 CO (ppmAfter Catalyst) 8 10 11 12 16 CO Removal Efficiency (%) 94.7 93.4 92.392.0 91.4 H₂CO (gm/BHP-hr 0.16 0.16 0.19 0.15 0.18 Before Catalyst) H₂CO(gm/BHP-hr After 0.015 0.016 0.019 0.015 0.020 Catalyst) H₂CO (ppmBefore Cat.) 23 23 27 20 25 H₂CO (ppm After Cat.) 2 2.3 2.9 2.0 2.7 H₂CORemoval 91.3 90.0 89.3 90.0 89.2 Efficiency (%) NO_(X) (gm/BHP-hr Before1.04 0.85 0.9 0.70 1.80 Catalyst) NO_(X) (gm/BHP-hr After 1.35 1.02 1.100.77 1.89 Catalyst) NO_(X) (ppm Before Cat.) 70 56 60 47 123 NO_(X) (ppmAfter Catalyst) 91 67 73 52 129 NO_(X) Increase Across 30.0 19.6 21.710.6 4.9 Catalyst (%)

While the preferred embodiments of the present invention have been shownin the accompanying figures and described above, it is not intended thatthese be taken to limit the scope of the present invention andmodifications thereof can be made by one skilled in the art withoutdeparting from the spirit of the present invention.

1. A silencer/catalytic converter unit for a 2-stroke natural gas fueledengine, the silencer/catalytic converter unit comprising: an oxidationcatalyst for reducing carbon monoxide and formaldehyde emissions in anuntreated engine exhaust; a first volume for dampening spurious exhaustpressure excursions and removing at least a portion of the particulatescontained in the untreated engine exhaust; a second volume housing theoxidation catalyst for reducing emissions in a treated engine exhaustbelow the emissions in the untreated engine exhaust; and a pressurerelief valve in communication with the first volume set to relieve at apressure greater than the maximum normal operating pressure of theengine exhaust to avoid excessive pressure excursions of the engineexhaust from damaging the oxidation catalyst, wherein the first volumeand the second volume are in communication with each other and whereinthe oxidation catalyst is positioned within the second chamber such thatduring operation of the engine the untreated engine exhaust has atemperature of at least 600 degrees F. at that position.
 2. The unitaccording to claim 1, wherein the oxidation catalyst having a firstcatalyst face, wherein the at least one exhaust flow pipe provides thecommunication between the first and second volumes, and wherein each ofthe at least one exhaust flow pipes has a catalyst facing end which isclosest to the first catalyst face and that the distance between thecatalyst facing end and the first catalyst face is sufficient to providea substantially uniform flow of the untreated exhaust upon contactacross the first catalyst face during engine operation.
 3. A lowemissions 2-stroke natural gas fueled engine, the engine comprising: atleast one cylinder with an inlet port and an exhaust port, asilencer/catalytic converter unit, wherein the silencer/catalyticconverter unit has at least: a first volume for dampening spuriousexhaust pressure excursions and removing at least a portion of theparticulates contained in an untreated engine exhaust, and a secondvolume housing an oxidation catalyst for reducing emissions in a treatedengine exhaust below the emissions in the untreated engine exhaust,wherein the first volume and the second volume are in communication witheach other; wherein the exhaust port is in fluid communication with thesilencer/converter; and a lubricating engine oil having a zinc contentof at most 10 ppm, thereby reducing the metal poisons contained in theuntreated exhaust prior to contact with the oxidation catalyst.
 4. Theengine according to claim 3, wherein the lubricating engine oil has azinc content of at most 5 ppm.
 5. The engine according to claim 3,wherein the lubricating engine oil has less than 0.1 wt % ash asdetermined by ASTM D874, thereby minimizing the amount of sulfurcombustion components contained in the untreated engine exhaust toreduce masking of the oxidation catalyst.
 6. The engine according toclaim 3, further comprising a pressure relief valve in communicationwith the first volume set to relieve at a pressure greater than themaximum normal operating pressure of the engine exhaust to avoidexcessive pressure excursions of the engine exhaust from damaging theoxidation catalyst.
 7. A method for converting an original 2-strokenatural gas fueled engine to a converted 2-stroke natural gas fueledengine, the method comprising the steps of: providing the original2-stroke natural gas fueled engine producing an untreated engine exhaustcontaining particulates, wherein the engine has at least: at least onecylinder with an inlet port and an exhaust port, a silencer, wherein theexhaust port is in fluid communication with the silencer; and anunmodified lubricating engine oil having a zinc content of at least 300ppm; replacing the silencer with a silencer/catalytic converter unit,wherein the silencer/catalytic converter unit has at least: a firstvolume for dampening spurious exhaust pressure excursions and removingat least a portion of the particulates contained in the untreated engineexhaust, and a second volume housing an oxidation catalyst for reducingemissions in a treated engine exhaust below the emissions contained inthe untreated engine exhaust, wherein the first volume and the secondvolume are in communication with each other; positioning the oxidationcatalyst within the second chamber such that the untreated engineexhaust has a temperature of at least 600 degrees F.; and replacing theunmodified lubricating engine oil with a low metals lubricating engineoil having at most 10 ppm zinc content, thereby reducing the metalpoisons contained in the untreated engine exhaust prior to contact withthe oxidation catalyst.
 8. The method according to claim 7, furthercomprising the step of: installing a pressure relief valve incommunication with the first volume set to relieve at a pressure greaterthan the maximum normal operating pressure of the engine exhaust toavoid excessive pressure excursions of the engine exhaust from damagingthe oxidation catalyst.
 9. The method according to claim 7, wherein thelubricating engine oil has a zinc content of at most 5 ppm.
 10. Themethod according to claim 7, wherein the low metals lubricating engineoil has less than 0.1 wt % ash as determined by ASTM D874, therebyminimizing the amount of sulfur combustion components contained in theuntreated engine exhaust to reduce masking of the oxidation catalyst.11. A method for reducing carbon monoxide and formaldehyde emissions inthe exhaust of a 2-stroke natural gas fueled engine, the methodcomprising: lubricating said engine with a lubricating engine oilcomposition having at most 10 ppm zinc; feeding an untreated engineexhaust of the engine to a silencer/converter to produce a treatedengine exhaust, wherein the silencer/converter has at least: a firstvolume for dampening spurious exhaust pressure excursions and removingat least a portion of the particulates contained in the untreated engineexhaust, and a second volume housing an oxidation catalyst for reducingemissions in the treated engine exhaust below the emissions in theuntreated engine exhaust, wherein the first volume and the second volumeare in communication with each other; and positioning the oxidationcatalyst within the second chamber such that the untreated engineexhaust has a temperature of at least 600 degrees F.
 12. The methodaccording to claim 11, wherein the lubricating engine oil has an ashcontent of less than 0.1 wt % according to ASTM D874, thereby minimizingthe amount of sulfur combustion components contained in the untreatedengine exhaust to reduce masking of the oxidation catalyst.
 13. Themethod according to claim 11, wherein the lubricating engine oil has azinc content of at most 5 ppm.
 14. The method according to claim 11,further comprising the step of: installing a pressure relief valve incommunication with the first volume set to relieve at a pressure greaterthan the maximum normal operating pressure of the engine exhaust toavoid excessive pressure excursions of the engine exhaust from damagingthe oxidation catalyst.